Various flocculants have hitherto been widely used for the flocculation/dewatering of sludges resulting from municipal sewage, industrial wastewaters, and the like.
For example, a method which comprises using poly(iron sulfate) as an inorganic flocculant, adding a nonionic, anionic, or cationic polymeric flocculent alone thereto to form flocs, and dehydrating the flocs (Japanese Patent Laid-Open No. 51998/1983) and a method of sludge dewatering which comprises using an inorganic flocculant and an amphoteric polymeric flocculant having a cationic nature and an anionic nature (Japanese Patent Laid-Open No. 16599/1984) have been proposed. Other techniques include a method comprising adding an inorganic flocculent, subsequently adjusting the pH to 5 to 8, and adding an amphoteric polymeric flocculant thereto (Japanese Patent Laid-Open No. 158200/1988).
Furthermore, known from long ago are a flocculent characterized by comprising a polymer salt consisting of a poly(carboxylic acid) which is partly or wholly in the form of a salt with a cationic surfactant having 6 or more carbon atoms (Japanese Patent Publication No. 1729/1967) and a flocculant comprising an anionic copolymer comprising as essential components acrylic acid and an alkyl acrylate, e.g., methyl acrylate (Japanese Patent Laid-Open No. 41281/1974). Moreover, a sludge dehydrant comprising an amphoteric copolymer comprising a cationic monomer, an anionic monomer, a water-soluble nonionic monomer, and a hydrophobic acrylic acid derivative having a solubility in water of 1 g or less, e.g., a (meth)acrylic acid alkyl ester having 8 or more carbon atoms, as essential components (Japanese Patent Laid-Open No. 156400/1999) and the like have recently been proposed.
However, there have been cases where even with such various flocculants and dewatering methods, a sufficient effect cannot be obtained depending on the kind of the sludge to be treated.
The techniques for flocculation/dewatering according to the various proposals described above have merits in their own ways and are used. However, with recent changes in the living environment and with the resultant increase in the amount of sludges from municipal sewage and industrial wastewaters, the amount of the flocculants and dehydrants to be used is increasing and this poses a problem. There is hence a strong desire for a flocculant or sludge dehydrant which, when used in a smaller amount, enables efficient flocculation/dewatering, i.e., which has better performance.
The present inventors made investigations in order to provide a flocculant and a method of sludge dewatering which, in sludge dewatering, give flocs having an excellent balance among flocculating strength, filtration rate, and moisture content and which can meet the desire.